Ink jet printing is a non-impact printing process in which an electronic signal produces droplets of ink that are deposited on a wide variety of substrates such as paper, transparent film, plastics, metals and fabrics. It is extremely versatile in substrate variety, print quality, speed of operation, and is digitally controllable. For these reasons, ink jet is widely used industrially for marking and labeling. In the office environment, it has been broadly utilized as output for personal computers. This is especially true for thermal or bubble jet drop-on-demand printers with disposable ink cartidges. These printers have been widely received due to their high print quality, low cost, relatively quiet operation and graphics capability.
Both dyes and pigments have been used as colorants for ink jet inks. However, dyes have several disadvantages. They are water-soluble and remain so after drying. They are redissolved by contact with water and will run when exposed to a water spill and smear on contact with felt pen markers. In addition, they exhibit poor light stability relative to pigments and are known to fade even under conditions of office fluorescent lighting. Many of these disadvantages prohibit the use of dye-based inks in applications requiring water-fastness and greater light stability.
Pigment-based inks contain cosolvents which provide improved water and smear resistance and improved lightfastness compared to dyes. However, the presence of these cosolvents (including penetrants) tends to destabilize pigment dispersions. Thus, pigments are a useful alternative to dyes provided the pigment dispersions can be made stable to flocculation and settling.
Three major issues dominate ink jet ink technology. They are (1) drying rate, (2) print quality, and (3) reliability. These issues compete in the product formulation process.
Drying rate determines the throughput rate and, therefore, productivity of a printer. One major deficiency of thermal ink jet printers is their relatively slow printing speed compared to printers using competing technologies. A major obstacle is the ink drying rate. Drying occurs by both evaporation and penetration. Evaporation is determined by solvent vapor pressure whereas penetration is determined by interfacial energy between the ink and the print media and the porosity of the print media. The use of penetrants as a means to increase the rate of penetration of inks is known in the art. However, many of the known penetrants cause pigment dispersions to flocculate and therefore cannot be used in pigmented ink jet inks.
Print quality can be somewhat subjective in measurement. Essentially, it is the image definition or contrast verses the nonimage background areas. It is controlled by two general factors:
1) Color properties of the ink which are measured by optical density and color coordinates controlling hue, e.g., L*, A* and B* in the CIELAB 1976 color standards. In the case of black inks, optical density is the most important factor.
2) Non-color image characteristics which determine the definition of the image are: a) resolution, i.e., number of drops of ink per unit area; b) the area coverage per drop, c) edge acuity; and d) presence or absence of artifacts such as satellite droplets, side pennants or feathering. Feathering in particular is an important phenomenon because it is often the result of a fast penetrating ink which conducts ink through paper fibers producing a fuzzy or feathery image.
An important reliability issue is decap or crust time which is defined as the time span over which a pen can be exposed to the atmosphere without failing to fire. Other reliability issues are stability of the ink caused by physical or chemical changes, compatibilty with equipment material, robustness towards variability in thermal firing conditions and stable drop volume over long term use.
In developing ink compositions, it is often necessary to sacrifice one or more of these properties to increase or improve the others. For example, methods for increasing drying rate will adversely affect either print quality or reliability. Using a more volatile cosolvent will decrease drying time but also decrease decap time. Using a more penetrating solvent will raise drying rate but will also lower optical density and induce feathering. Increasing optical density by employing more colorant can adversely effect reliability. Faster penetrating cosolvent improve the drying rate, but many of them will destabilize pigment dispersions.
Accordingly, a need exists for penetrants that will increase ink drying without degrading print quality through feathering and without destabilizing pigment-based inks.
The penetrants of this invention are able to increase the penetration rate thereby increasing drying rate without inducing feathering or reducing decap time or decreasing pigment dispersion stability . These penetrants are also useful in dye-based inks.